A number of conventional wireless communication techniques have been developed. One common technique is code division multiple access (CDMA) in which multiple communications are simultaneously conducted over a radio-frequency (RF) spectrum. Example wireless communication devices (“subscriber units”) that have incorporated CDMA technology include cellular radiotelephones, satellite radiotelephones, PCMCIA cards for portable computers, personal digital assistants (PDAs) equipped with wireless communication capabilities, and the like.
A CDMA system may be designed to support one or more CDMA standards such as (1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the “TIA/EIA-98-C Recommended Minimum Standard for Dual-Mode Wideband Spread Spectrum Cellular Mobile Station” (the IS-98 standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the WCDMA standard), (4) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in a set of documents including “TR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems,” the “C.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,” and the “C.S0024 CDMA2000 High Rate Packet Data Air Interface Specification” (the CDMA2000 standard), and (5) some other standards. A system that implements the High Rate Packet Data specification of the CDMA2000 standard is referred to herein as a high data rate (HDR) system. The HDR system is documented in TIA/EIA-IS-856, “CDMA2000 High Rate Packet Data Air Interface Specification.” Proposed wireless systems also provide a combination of HDR and low data rate services (such as voice and fax services) using a common air interface.
A typical CDMA communication system includes a number of mobile subscriber units that encode voice and data in CDMA waveforms. The subscriber units communicate with base stations, also referred to as base transceiver subsystems (BTS), cell stations, cell sites, or simply cells. A base station demodulates incoming CDMA waveforms received from subscriber units within a limited geographic region, and transmits outgoing CDMA waveforms to the subscriber units. A base station controller (BSC) provides an interface between the base stations and the public switched telephone network (PSTN) for routing the signals to other remote base stations or to any conventional telephony system. In general, transmission from the base station to the subscriber unit is referred to as the Forward CDMA Channel, and is sometimes called a downlink. Transmission from the subscriber unit to the base station is referred to as the Reverse CDMA Channel, and is sometimes called an uplink.
At any given moment, a single subscriber unit or base station may receive multiple replicas of the transmit signal, each replica possibly having a different amplitude, phase and time delay. The replicas are often caused by reflections of the transmit signal off of obstacles in the environment, such as buildings, trees, cars and people. The various replicas of the transmit signal are referred to as “paths,” with the general characteristic referred to as “multipath.”
The transmit signal of a CDMA communication system consists of a train of pulses called “chips.” More specifically, a transmitter generates a spread spectrum transmit signal by modulating an outbound serial stream of data with a pseudonoise (PN) code. The application of the PN code to the data produces a stream of chips. The resulting chips are transmitted according to some modulation scheme, such as quadrature phase shift keying (QPSK) modulation. In order to separate signals from multiple users, the receivers isolate the signal of the desired user by matching the signal to the corresponding PN code.
The rate that the PN code is applied is referred to as the chip rate, which is typically many times faster than the data rate. The duration of one pulse of the PN code is often used as a unit of measure referred to as chip time. The time delay between multiple received paths is often represented in chip time. A time delay of 1.5 chips between two paths, for example, indicates a time delay equal to 1.5 times the chip time of the transmit signal.
In order to reduce effects of signal fading and for other advantages, conventional CDMA receivers discriminate between multiple paths during demodulation. In particular, a typical CDMA receiver, commonly referred to as a RAKE receiver, contains a number of demodulation elements. When the paths have a time separation greater than a threshold, such as approximately 1.5 chips or more, the receiver typically assigns the demodulation elements time offsets corresponding to the different paths. For shorter time separations, often referred to as “unresolvable multipath spacing,” conventional receivers typically do not assign multiple demodulation elements because the multiple paths often appear as a single mass of energy without readily detectable peaks. Similarly, if multiple paths shift from greater time separations to unresolvable multipath, the demodulation elements often converge to have the same time offset. When two demodulations elements have “merged” as such, conventional receivers typically deassign one of the demodulation elements.